Investigation of Historic 

Equilibrium Moisture 

Content Data from 

the Forest Products 

Laboratory

Samuel V. Glass

Samuel L. Zelinka

Jay A. Johnson

Forest

Products

Laboratory

General

Technical 

Report

FPL–GTR–229

United States Department of Agriculture

Forest Service
January 2014

Glass, Samuel V.; Zelinka, Samuel L.; Johnson, Jay A. 2014. Investigation of historic equilibrium moisture content data from the Forest Products 

Laboratory. General Technical Report FPL–GTR–229. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 34 p.

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Abstract

The Forest Products Laboratory (FPL) has provided 

equilibrium moisture content (EMC) values of wood for 

given temperature and relative humidity (RH) conditions 

in various forms over the course of its history, primarily for 

practical purposes related to drying lumber and controlling 

moisture content. The FPL EMC data have been widely 

cited and reprinted, not only in literature of a practical 

nature such as the Wood Handbook, but also in textbooks 

and journal articles as a basis for scientific discourse on the 

thermodynamics of water vapor sorption in wood and evaluation of physical models. Using the data for such scientific 

purposes presupposes that the methods by which the data 

were acquired are well documented and accepted. This 

report questions previous assumptions about the historic 

EMC data and attempts to uncover and evaluate the original 

data sources. It also addresses a number of related topics, 

including how the presentation of data has evolved in the 

literature and whether the data are practically applicable to 

all wood species. We find that the data are unreliable for 

scientific purposes, such as thermodynamic analysis and 

testing of physical models, for three reasons: lack of proper 

documentation of methodology; the unsolvable problem of 

knowing which values are determined from direct observations and which are interpolated; and the absence of definitive measurement error analysis. However, we affirm that 

the data are indeed useful for practical applications, such 

as lumber drying, conditioning of wood specimens prior to 

testing physical or mechanical properties, and modeling of 

moisture content of wood members in buildings. We show 

that a number of mathematical models provide adequate fits 

to the data for practical use.

Keywords: Equilibrium moisture content, hygroscopicity, 

sorption, Wood Handbook, kiln drying, air drying, 

seasoning, conditioning, mathematical model

Acknowledgments

We are grateful to William Simpson, Jieying Wang, and the 

late Charles Carll for critical reviews of this manuscript. 

We also thank John Hermanson and Alex Wiedenhoeft for 

valuable discussions and Julie Blankenburg for assistance in 

locating numerous reports.

Contents

1. Introduction................................................................. 1

2. Sources of Data ........................................................... 2

 2.1 H.E. McKenzie...................................................... 2

 2.2 M.E. Dunlap.......................................................... 2

 2.3 C.A. Menzel.......................................................... 4

 2.4 W.K. Loughborough.............................................. 5

 2.5 Summary of Data Sources .................................... 9

3. Metamorphosis............................................................

 3.1 Table of EMC Values at Room Temperature ........ 9

 3.2 Table and Figure: EMC Values at Three 

 Temperatures..............................................................10

 3.3 Preliminary Figure and Table with Broad 

 Temperature Range ....................................................13

 3.4 EMC and RH Figures and Tables from 30 °F to 

 210 °F and Beyond ....................................................14

 3.5 Summary of Metamorphosis.................................23

4. Are the Data Valid for Scientific Purposes?...............23

5. Practical Applicability of the Data.............................23

 5.1 Claims Regarding Wood Species in FPL 

 Literature....................................................................25

 5.2 Perspectives on Sorption Hysteresis in FPL 

 Literature....................................................................26

 5.3 Comparison with Literature Data for Southern 

 Yellow Pine ................................................................26

 5.4 Comparison with Literature Data for Different 

 Wood Speices.............................................................27

 6. Mathematical Formulations for Practical Use ..........27

7. Conclusions................................................................30

References......................................................................31
Investigation of Historic Equilibrium 

Moisture Content Data from the 

Forest Products Laboratory

Samuel V. Glass, Research Physical Scientist

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Jay A. Johnson, Professor Emeritus

University of Washington, School of Forest Resources, Seattle, Washington

1. Introduction

The Forest Products Laboratory (FPL) has provided equilibrium moisture content (EMC) values of wood for given temperature and relative humidity (RH) conditions in various 

forms over the course of its history, having published data 

as early as 1919 (Koehler 1919; FPL 1919a,b,c). These data 

have primarily served practical, technological purposes related to drying lumber and controlling moisture content. The 

importance of the FPL EMC data can be appreciated by considering how widely they have been reprinted in literature 

of a practical nature: the data appear in the Wood Handbook

(FPL 1935, 1940, 1955, 1974, 1987, 1999b, 2010); in publications relating to the use of wood in aircraft construction 

(FPL 1919c, 1928, 1941a, 1942, 1943, 1944, 1946, 1951; 

U.S. Army Air Forces 1944; Voorhies and Loughborough 

1943); in wood drying manuals such as the Dry Kiln Operator’s Manual (Rasmussen 1961; Simpson 1991), Air Drying 

of Lumber (FPL 1999a), and Drying Hardwood Lumber

(Denig et al. 2000); and in international standards, particularly the Standard Guide for Moisture Conditioning of Wood 

and Wood-Based Materials (ASTM 2010). The Forest Products Laboratory and other organizations in North America 

have long considered the FPL EMC data set to be the first 

place to go for such data.

In addition to their frequent appearance in documents of a 

practical nature, the FPL EMC data have been cited in scientific journals and textbooks as a basis for scientific discourse 

on the thermodynamics of water vapor sorption in wood 

(Stamm and Loughborough 1935; Stamm 1964; Kollman 

and Côté 1968; Skaar 1988; Siau 1995; Keey et al. 2000). 

In addition, several journal articles (Simpson 1971, 1973, 

1980) have evaluated the ability of mathematical models, 

some of which were derived from physical principles, to 

fit the FPL data. Using the data for such scientific purposes 

presupposes that the data are of sufficient quality and that 

the methods by which the data were acquired are well 

documented and accepted. Is this actually the case?

Although the FPL EMC data have been cited extensively, 

their origins are obscure. Several FPL publications have 

in fact urged caution regarding general use of the data. 

Simpson (1973), for example, cautioned that the data do 

not represent either adsorption or desorption in the usual 

sense. He identified the species used in the original measurements as Sitka spruce and referred the reader to Stamm 

and Loughborough (1935) for experimental details. Simpson 

and Rosen (1981) made similar comments on the source of 

the data and stated, “Although thorough documentation is 

not available, the authors understand that the data of several 

other species are also represented. Despite these imperfections, the data have served their practical purpose well for 

many years.” More recently, Denig et al. (2000) commented, 

“These standard EMC data, which were collected more than 

50 years ago, are primarily based on the drying of small 

shavings of Sitka spruce in a dryer with a small oscillation 

in RH. In spite of the potential shortcomings of the data, 

they serve very well for drying hardwood lumber.”

We are primarily interested in answering the question of 

whether the methods by which the FPL EMC data were obtained are sufficiently well documented to allow the data to 

be used for scientific purposes such as thermodynamic analysis and evaluation and validation of physical models. The 

fact that data serve well for practical purposes does not necessarily imply that the methods by which they were acquired 

are documented and accepted for such scientific purposes. 

In this review, we question previous assumptions about the 

historic EMC data and attempt to uncover and evaluate the 

original data sources. In the process, we raise a number 

of related questions about the data: how and why has the 

presentation of data evolved in the literature? Are the data 

practically applicable to all wood species? What have FPL 

publications said in the past regarding sorption hysteresis? 

Is the level of precision given by FPL publications on mathematical modeling of EMC appropriate?
General Technical Report FPL–GTR–229

2

2. Sources of Data

2.1 H.E. McKenzie

H.E. McKenzie (n.d.) conducted two series of water vapor 

sorption experiments at the Yale Forest School and filed an 

FPL progress report reviewing the literature and presenting 

his results. The date of his experiments and report, though 

uncertain, are definitely prior to 1917 because the report was 

cited by Tiemann (1917).1 Both series of experiments were 

conducted near 212 °F (100 °C).

The first series of experiments included spruce, longleaf 

pine, red oak, and white oak. The documentation of experimental details is poor. Apparently only one specimen was 

measured for each species, with dimensions 50 mm by 

50 mm by 13 mm (the orientation with respect to grain direction was not stated). Specimens were exposed to five different relative humidity levels: 25%, 33%, 50%, 66%, and 

100% RH. Humidity values less than 100% were attained 

using vacuum. The initial condition of the specimens (green 

or dry) and the order of measurements are not stated. Further, it is unclear whether specimens were oven-dried before 

or after they were conditioned at various RH levels. However, the EMC data are presented in order of decreasing RH 

(desorption). Finally, it is questionable whether equilibrium 

values were actually attained, as mass readings were taken 

just a few hours after the relative humidity was changed; no 

data are shown to indicate whether specimen mass became 

stable over this short time.

McKenzie did not regard the measurements at 100% RH as 

equilibrium values: “It was determined by later investigations, however, that a true equilibrium between a saturated 

vapor and a moisture-holding substance is impossible.” 

These later investigations involved monitoring specimen 

mass with time at 100% RH: “...the absorption process appears to be unending though its velocity continuously 

diminishes.”

The second series of experiments included spruce, southern 

yellow pine (identified as longleaf pine in the report text 

but as loblolly pine in the data table), tulip, hickory, blue 

gum, and white ash. The author does not describe specimen 

dimensions, the number of specimens, the initial condition, 

or the order of measurements. A slightly different range of 

RH levels was used: 17%, 33%, 50%, 66%, and 84% RH 

for spruce and pine; 17%, 33%, 50%, and 66% RH for tulip; 

and 33%, 50%, 66%, and 82% RH for hickory, blue gum, 

and white ash.

1Tiemann (1924) dated McKenzie’s work as ca. 1908. However, the report 

had to have been written in 1910 or later because 1) McKenzie states in the 

introduction, “A careful search for work done along these lines by other 

investigators has been made through ‘Science Abstracts’, covering a period 

of fifteen years (1896 to 1910),” and 2) the Forest Products Laboratory did 

not exist prior to 1910. The FPL library catalog gives an estimated date of 

ca. 1912–1915. McKenzie may have been done the experiments at Yale 

prior to joining FPL.

Figure 1 plots the values from both of McKenzie’s data tables. Given that the methodology was largely undocumented 

and probably did not include replicate measurements, this 

data set is of questionable reliability.

2.2 M.E. Dunlap

2.2.1 Methodology

Matthew E. Dunlap (1919) conducted a series of sorption 

experiments at room temperature with seven wood species 

“taken from laboratory stock”: white oak, black walnut, 

Sitka spruce, yellow birch, ash, mahogany (African), and 

longleaf pine (2.4% and 25.0% resin content). Specimens 

were soaked for several hours, and shavings 13 mm by 

140 mm were cut with a hand plane. These shavings were 

strung to copper wires (Fig. 2) so that specimens could 

be weighed in situ inside a sealed chamber (Fig. 3). The 

chamber also included a dew point hygrometer, dry-bulb 

thermometer, and fan. Temperature and humidity inside the 

chamber were not actively controlled. Temperatures were 

between 75 °F (24 °C) and 86 °F (30 °C), with an average 

near 80 °F (27 °C). Two matched specimens of a given species were placed in the chamber at a time, one at a slightly 

lower MC than the other. The relative humidity was thus 

regulated by the hygroscopicity of the specimens, whose 

moisture contents (MCs) tended to approach an intermediate 

value. EMC was defined as the average MC of the two specimens after 24 h. Dry-bulb and dew point temperatures were 

also recorded after 24 h (from which RH was determined). 

For the next step, one specimen was dried by a few percent 

MC, and then both specimens were placed in the chamber to 

0

2

4

6

8

10

12

14

16

20 40 60 80 100

Equilibrium moisture content (%)

Relative humidity (%)

Spruce I

Spruce II

Longleaf pine

Loblolly pine

Red oak

White oak

Tulip

Hickory

Blue gum

White ash

0

Figure 1. EMC data from McKenzie (n.d.).
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

3

equilibrate at a slightly lower EMC than the previous step. 

Because of the drying and resorption of one of the specimens, we refer to this method as “quasidesorption.” This process was repeated to give EMC values 

over a range from 100% RH down to about 20% RH. After 

all the measurements were completed, the specimens were 

dried to constant weight at 212 °F (100 °C).

2.2.2 Findings, Caution, Call for Further Work, 

and Speculation

Dunlap’s results are shown in Figure 4 together with the 

average of all species from 20% to 100% RH in 5% RH 

increments, as reported by Dunlap.

Dunlap advised that his data were not to be used at high 

temperatures: “The results of this work should be applied 

only to conditions at approximately 65 to 85 °F. They do 

not apply directly to kiln operations where higher temperatures are used. We have not obtained any information on the 

moisture content of wood at the same humidities but higher 

temperatures, but it has been observed in dry kiln operation 

that lower values for the moisture content at a given humidity are obtained when higher temperatures are used.” Dunlap 

wisely recommended further work in this area: “It would be 

extremely valuable in kiln drying operations to have similar 

data obtained at temperatures of say 120 °F, 140 °F, 160 °F, 

and 180 °F.”

Regarding differences between species, Dunlap exhibited 

less caution: “It is interesting to note that there is comparatively little variation in the values obtained for the individual species tested and in a general way it is thought that the 

average curves would apply practically to any wood.” Although Dunlap is correct that the species he studied showed 

little variation, it does not follow logically that little variation in EMC exists among all wood species. However, this 

unjustified speculation is balanced by a statement that 

“...work carried out at room temperatures on additional 

species would be of value.”

2.2.3 Critique

Comparing Dunlap’s progress report with McKenzie’s, we 

observe that Dunlap’s methodology is far more clearly documented, that Dunlap recorded measurements at a greater 

number of RH levels, and that Dunlap’s data show much 

smaller between-species variation. Although the improvements are considerable, we find several methodological 

problems in Dunlap’s experiments:

•	 RH values are reported to a precision of 0.1% RH, but 

there is no estimate of error in RH (which would be 

based on error in dry-bulb and dew point temperatures).

•	 No data are presented to justify that 24 h was sufficient 

for equilibration. Nothing is reported regarding the 

Figure 2. Specimens used in sorption experiments by 

Dunlap (1919).

Figure 3. Apparatus used in sorption experiments by 

Dunlap (1919).

5

10

15

20

25

30

35

40

0 20 40 60 80 100

E

quillibrium

m

oisture content (%)

Relative humidity (%)

White oak, 80°F

Black walnut, 84°F

Sitka spruce, 76–86°F

Yellow birch, 75–85°F

Ash, 76°F

Mahogany (African), 82°F

Longleaf pine (2.4% resin), 85°F

Longleaf pine (25.0% resin), 85°F

Reported average

0

Figure 4. EMC data from Dunlap (1919).
General Technical Report FPL–GTR–229

4

magnitude of the difference in MC between the two 

specimens that were averaged to determine EMC after 

24 h. However, the thin wood shavings used in the experiments would be expected to equilibrate much more 

rapidly than wood blocks, and the average of the two 

specimens (one losing moisture and the other gaining 

moisture) would not be expected to change considerably over time.

•	 A number of measurements are reported at 100% RH; 

however, the report does not explain how this condition 

was achieved. McKenzie (n.d.) had previously rejected 

measurements at 100% RH because equilibrium could 

not be established. As might be expected, the variability 

in Dunlap’s data is considerably higher for values at 

100% RH than for values at lower RH; this is shown by 

the following analysis. For all values reported at 100% 

RH, the standard deviation is 2.9 percentage points (denoted % MC). By contrast, a three-parameter parabolic 

fit (Eq. (1)) to all the data yields a root-mean-square 

error (RMSE) of 1.5% MC. When data points with 

RH ≥ 98% are excluded from the fit, the RMSE drops 

to 1.0% MC. The form of the fitting equation is

(1)

where m is equilibrium moisture content (decimal) and

h is relative humidity (decimal).

2.3 C.A. Menzel

Carl A. Menzel (1921) investigated moisture content and 

shrinkage of red oak at 80 °F (27 °C). Introductory remarks 

state that although no extensive tests were possible (data 

were acquired as time and apparatus permitted), information 

was obtained that would be valuable in planning a thorough 

investigation of the hygroscopic and shrinkage properties 

of wood. Prior to beginning this study, Menzel had noticed 

that wood of various species placed in different conditioning 

rooms in the “Propeller Laboratory” displayed great differences between species and within species in regard to rate of 

change in moisture content as well as equilibrium moisture 

content. This research had an obvious practical application 

in aircraft design and construction, which is explored further 

in Section 3.

2.3.1 Methodology

Specimens were taken from 1-m-long sticks of air-dried red 

oak, both plain and quarter sawn, with a wide range in specific gravity. These were cut to 25 mm in length along the 

grain and planed to 50 mm in width and 22 mm in thickness. 

Initial moisture content was about 10%. A total of 46 specimens were used for EMC measurements.

Specimens were conditioned in rooms maintained at 80 °F 

(27 °C) and at RH levels of approximately 30%, 60%, and 

90%. A 100% RH condition was attained by placing blocks 

in a large closed stone jar containing water (though this is 

not relevant to the EMC data; dimensions were measured to 

determine shrinkage values). Relative humidity in the conditioning rooms was determined from dry- and wet-bulb thermometers read to within 0.5 °F (0.3 °C). Specimens were 

weighed to 0.001 g with an analytical balance. Specimens 

were divided into two groups of 23 each and subjected to 

different conditioning sequences:

1. 30%–60%–90%–60%–30%–oven 

dried–30%–60%–90%–100%–soaked in water–oven 

dried–30%–60%–90%–oven dried

2. 30%–60%–90%–60%–oven dried–30%–60%

The amount of time at each humidity condition varied from 

16 to 50 days. Successive weighing showed small fluctuations over time that may have been caused by humidity fluctuations. The author apparently assumed that the specimens 

had reached equilibrium.

2.3.2 Findings, Interpretation, and Speculation

Figure 5 shows Menzel’s EMC data as a function of ovendry specific gravity (equivalent to oven-dry density expressed in g cm–3) for the second group of 23 specimens. 

Clearly EMC is not correlated with specimen density. 

Technical Note 89 (FPL 1920) provides a summary table 

of Menzel’s EMC and density data for nine representative 

specimens.

A fair degree of scatter is evident in the data. Menzel 

pointed out that the variability is higher at the low and high 

RH levels than at intermediate RH levels. Standard deviations are 0.6%, 0.3%, and 0.7% MC at relative humidities 

of 38%, 61%, and 88% RH, respectively. Menzel referred 

to other (apparently unpublished) experiments he had done 

that showed the opposite for hard maple—lower variability 

at the humidity extremes.2 He also states, “In general other 

species show small variations in moisture content at low 

humidities, growing greater with increase of relative humidity.” He ascribes variability in hygroscopicity between 

different species and within the same species to “different 

proportions of various hygroscopic substances in the wood, 

each one of which has a distinct moisture humidity curve.” 

Although he did not specifically name cellulose, hemicelluloses, and lignin, Menzel clearly suggested that variation 

in EMC stems from variation in wood chemistry. This is the 

first instance of this argument in FPL literature, to the best 

of our knowledge. Later research showed that these classes 

of wood polymers do, in fact, differ in hygroscopicity, hemicelluloses being the most hygroscopic and lignin the least 

hygroscopic (Christensen and Kelsey 1958).

Menzel found that the oven-dry mass decreased upon successive dryings: specimens lost an average of 1.35% of their 

2Tiemann (1920) includes a figure showing EMC data for maple and birch 

at 80 °F (27 °C), which he attributes to Menzel, in addition to the data for 

red oak.

m = h

Ah2 + Bh + C
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

5

mass between the first and second dryings but only 0.08% 

between the second and third dryings. Menzel ascribed this 

to a loss of volatile matter. He noted that the first oven drying was done following conditioning at 30% RH, the second 

following soaking in water, and the third following conditioning at 90% RH. He suggested that volatile matter is lost 

more readily when wood is oven dried from a high moisture 

content than from a low moisture content because some 

volatile constituents are dissolved and carried away when 

water is driven off.

Menzel also found that the EMC curve for “reabsorption” 

after oven drying was about 2% MC lower on average than 

the curve prior to oven drying, based on the same first ovendry mass. He did not offer any suggestion as to why this 

occurs. Menzel fit the EMC data with parabolic functions, 

which he extrapolated to 100% RH as a theoretical fiber 

saturation point (FSP). He obtained a value of 23.6% MC 

for red oak that had not been oven dried, and 21.4% for red 

oak after the first oven drying.

Tiemann (1922) reviewed Menzel’s work, praising the accuracy and extent of his measurements but disagreeing with 

some interpretations. Tiemann argued that the mass loss 

between the first and second oven dryings was a result of 

extraction rather than vaporization; specimens had been 

soaked in water at 80 °F (27 °C), and soluble materials had 

likely leached out. Tiemann also cited unpublished data on 

yellow birch blocks that showed little difference in oven-dry 

mass after drying from a low moisture content as compared 

to drying from a high moisture content. Tiemann suggested 

that EMCs on “reabsorption” after oven drying may be 

lower than those before oven drying because of loss of 

volatile oils in the oven or reduction in hygroscopicity. The 

concept of sorption hysteresis was not mentioned. Tiemann 

noted that Menzel’s data for red oak prior to oven drying 

corresponded very closely with Dunlap’s data for Sitka 

spruce. This close correspondence can be seen in Figure 6.

2.3.3 Critique

In general, Menzel’s measurements are well documented. 

He describes the number of specimens, their source, and the 

conditions to which they were exposed. Specimen mass was 

measured with high precision. However, the number of RH 

levels was limited (30%, 60%, 90%), RH conditions fluctuated considerably, and RH measurements lacked precision. 

The stated uncertainty in dry- and wet-bulb temperature 

measurements, ±0.5 °F (±0.3 °C), implies an RH uncertainty 

of approximately ±4% RH.

2.4 W.K. Loughborough

2.4.1 EMC Data: Published yet Concealed

The practical need for EMC data over a broader temperature 

range, as recommended by Dunlap (1919), may have been 

the motivation for new FPL measurements. These data first 

appear in several publications in the 1930s. From what follows below, it is clear that W. Karl Loughborough was responsible for the measurements and completed them prior 

to 1931. However, there is no FPL report that clearly 

documents the actual measurements, to the best of our 

knowledge.

0

5

10

15

20

25

0.5 0.6 0.7 0.8

E

quilibrium

m

oisture content (%)

Oven-dry specific gravity

88% RH

61% RH

38% RH

Figure 5. EMC versus oven-dry specific gravity for 23 red 

oak specimens (Menzel 1921).

5

10

15

20

25

30

35

0 20 40 60 80 100

E

quillibrium

m

oisture content (%)

Relative humidity (%)

Sitka spruce (Dunlap 1919)

Red oak Series I (Menzel 1921)

Red oak Series II (Menzel 1921)

0

Figure 6. Comparison of EMC data at approximately 80 °F 

(27 °C) for Sitka spruce shavings (Dunlap 1919) and red oak 

blocks (Menzel 1921).
General Technical Report FPL–GTR–229

6

The data were first published in an FPL technical bulletin 

entitled Wood-Liquid Relations (Hawley 1931).3 This bulletin presents the data in a figure showing EMC values 

for Sitka spruce from 70 °F (21 °C) to 212 °F (100 °C), 

reprinted here as Figure 7. Although Hawley (1931) notes, 

“The data represented in these curves are the work of W.K. 

Loughborough,” there is no reference to published work, 

and the text is silent regarding the measurement methodology. The text does state, “The solid lines in the curves ... are 

based on experimental data; the dotted parts of the lines are 

calculated.” However, the calculation method for the dotted 

parts of the curves is not described.

The same figure appears in journal articles by Loughborough and Rietz (1932) and by Stamm and Loughborough 

(1935).

2.4.2 Methodology

Although the emphasis of the article by Stamm and Loughborough (1935) is thermodynamic analysis, they do provide 

a brief description of the experiments.

Specimens were 3-mm-thick strips of green flat-sawn Sitka 

spruce. Specimen oven-dry mass was between 100 g and 

125 g (presumably determined after EMC measurements 

were completed). The number of specimens measured at a 

given temperature and humidity condition ranged from 25 to 

40. Specimens were suspended from lead stoppers located in 

the roof of a chamber and were weighed in situ.

The apparatus consisted of four chambers, each with a 

volume of about 0.45 m3. An electric blower moved the air 

within the chambers at a velocity of about 0.5 m s–1. Drybulb temperature and humidity (as wet-bulb temperature) 

were controlled with thermostats. Conditions were read 

frequently through glass windows. It was noted that both 

dry- and wet-bulb temperatures oscillated, with average 

amplitudes of 0.25 °F (0.14 °C) and 0.5 °F (0.3 °C), respectively. The dry- and wet-bulb temperatures would sometimes be out of phase, such that the wet-bulb depression was 

sometimes 0.75 °F (0.4 °C) greater or less than desired. This 

resulted in relative humidity oscillations in the range from 

3Loughborough was clearly involved in the preparation of this technical 

bulletin authored by Hawley (1931), which includes the following note: 

“The difficult problem presented by the movement of moisture in wood, 

which has been unsolved for years, required the best thought of specialists 

and many points of view, and successful and efficient work demanded 

close coordination of the efforts of several investigators. The moisture 

committee of the Forest Products Laboratory was formed to obtain such 

coordination within the laboratory. The personnel of the committee has 

included: E. Bateman, F.L. Browne, M.E. Dunlap, L.F. Hawley (chairman), 

A. Koehler, W.K. Loughborough, A.J. Stamm, R. Thelen, H.D. Tiemann, 

and F. Tuttle. The wide and varied experience of the members has 

supplied a comprehensive foundation for the investigative and analytical 

work of the moisture committee and has therefore contributed largely to 

this initial report. The sub-committee on publication, Messrs. Bateman, 

Loughborough (chairman), and Tiemann, has been of special assistance 

in the study of the literature and in the preparation of the report itself. The 

author is pleased to acknowledge his indebtedness to his associates.”

1.5% to 3% at higher temperatures and from 4% to 8% at 

lower temperatures. EMC measurements were made in steps 

of successively lower RH. The method is thus called “oscillating vapor pressure desorption.” Unfortunately, the period 

of oscillation and the time required to reach equilibrium 

were not mentioned. Nothing was said regarding the criterion for defining equilibrium (e.g., change in mass less than 

some threshold over a certain time period).

Stamm and Loughborough (1935) claimed that this method 

practically eliminated sorption hysteresis: “The hysteresis 

phenomenon encountered in desorption and adsorption measurements made on very small specimens under the most 

carefully controlled vapor pressure conditions was largely 

eliminated in these measurements. The specimens used were 

sufficiently large to permit the setting up of moisture gradients across the sections during the process of drying. This 

combined with the oscillations in vapor pressure permit alternate desorption and adsorption to take place, thus tending 

to establish an intermediate moisture content equilibrium.”

To illustrate their point, Stamm and Loughborough (1935) 

compared their data taken under oscillating vapor pressure 

desorption conditions with data acquired under “carefully 

controlled conditions [that] tend to promote the maximum 

hysteresis effect.” Figure 8 shows this comparison. The 

desorption and adsorption boundary curves were obtained 

using small cross sections of Sitka spruce weighed in situ

under tight thermostat control (25 °C ± 0.02 °C; 77 °F 

± 0.04 °F) and stable RH, regulated by saturated salt solutions (Seborg and Stamm 1931). Stamm and Loughborough 

(1935) claimed, “All the available drying data obtained 

under industrial control conditions on other softwood species also gave curves which fell within this hysteresis loop.” 

However, no published data on other softwood species were 

cited. The data were presumably unpublished measurements 

from FPL.

2.4.3 Critique

Measurement of moisture content at 100% RH and extrapolation of measured values to 100% RH are both problematic. Regarding measurement, it is practically impossible to 

achieve equilibrium at 100% RH. This was noted earlier by 

McKenzie (n.d.). Advancing his line of thought, we argue 

that 100% RH does not correspond to a unique equilibrium 

moisture content because 100% RH by definition means that 

water in wood has the same vapor pressure as bulk liquid 

water. A range of moisture contents exist that can satisfy this 

definition, from some lower bound at which liquid water is 

first present up to maximum saturation.

Hawley (1931) argued that FSP cannot be accurately determined by extrapolation of the sorption isotherm to 100% 

RH because of capillary condensation:

“The points where the temperature curves of Figure 

[7] intersect the 100 per cent relative-humidity curve 
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

7

Figure 7. EMC curves based on Loughborough’s data (Hawley 1931). The original figure caption reads, “The moisture-content values of Sitka spruce at equilibrium with various temperatures, partial vapor pressures, and relative humidities.”
General Technical Report FPL–GTR–229

8

indicate the fiber-saturation points at the respective temperatures. Most of these points were not actually determined—as indicated by the dotted portions, most of the 

temperature curves were extrapolated (by calculation) 

above the 90 per cent relative humidity intersections. 

The fiber-saturation point can not be accurately and directly determined by the method of Figure [7] because 

of the small capillaries (the cell cavities) existing in the 

wood. In a saturated atmosphere these cavities will condense moisture and consequently will give an apparent 

fiber-saturation point much higher than the actual value 

that is due to the water absorbed by the cell wall.”

Despite these considerations, EMC measurements are 

shown in Figure 7 as solid curves (implying they were 

measured values) at 100% RH for temperatures of 70 °F 

(21 °C), 80 °F (27 °C), and 90 °F (32 °C). Figure 8 also includes a data point at 100% RH (equivalent to a relative 

vapor pressure of 1.0) and 25 °C (77 °F). Stamm and 

Loughborough (1935) never explained how equilibrium at 

100% RH was achieved. As noted above, the calculation 

method for extrapolating the dotted parts of the curves in 

Figure 7 was not described. Moreover, Stamm and Loughborough (1935) referred to the extrapolated values at 100% 

RH for various temperatures as the fiber saturation point.

Stamm (1971) later commented on the determination of fiber saturation points by extrapolating sorption isotherms. In 

addition to the fact that different points are reached by extrapolating adsorption and desorption curves, Stamm noted 

that strictly speaking, extrapolation to 100% is incorrect 

because of capillary condensation. However, he argued that 

extrapolation from data points at roughly 97% or 98% RH 

is justifiable on the following grounds. The largest possible 

void in the cell wall was considered to be about 200 nm, 

and by the Kelvin equation, capillary condensation would 

occur at 99.5% RH. The difference between extrapolating 

to 99.5% RH versus 100% RH is practically negligible provided the curve does not change drastically between 97% or 

98% RH and 99.5% or 100% RH. Stamm claimed that 

this is the case for wood on the basis of pressure plate 

measurements.

From our perspective, the most serious problem with 

Loughborough’s data is that the actual data points are concealed. With the exception of Figure 8, the EMC data do 

not appear as discrete points in graphical or tabular form 

(though EMC tables appear in later FPL literature; see 

Section 3.4 below). The following questions cannot be 

answered from the information provided by Stamm and 

Loughborough (1935) or by any other documentation to the 

best of our knowledge:

•	 How many data points were measured at a given temperature? Were EMCs determined at increments of 10% RH 

at each temperature, as suggested by Figure 8 (relative 

vapor pressure increments of 0.1)? 

•	 Were measurements taken at all the temperatures 

shown in Figure 7, or were some of the curves drawn 

by interpolation?

•	 What is the variability across replicate specimens 

under a given condition?

Menzel (1921) had previously found considerable variability in EMC of red oak specimens (see Fig. 5); standard 

deviation varied with RH level and was in the range 0.3–

0.7% MC. In some cases, the estimated error in EMC determination may be less than the variability across specimens. 

For Loughborough’s data, the measurement error can be estimated as approximately ±0.1% MC based on the reported 

weighing accuracy (0.05 g) and the specimen size (~100 g 

oven-dry mass). However, we do find evidence elsewhere 

that Loughborough considered measurement error to be 

larger than variability.

An “Office Memorandum” written by Loughborough (1930) 

discusses errors in moisture content measurement by standard laboratory oven-drying procedures. A large number of 

yellow birch specimens had been conditioned, used in mechanical tests, and oven dried to determine moisture content. 

The resulting mean was 10.4% MC with a standard deviation of 0.65% MC. The methodology was clearly different 

from that used in the study on Sitka spruce; however, the 

assumptions involved in the analysis are revealing. Notably, 

it was assumed that EMC does not vary across specimens: 

“Because all the material had approximately the same history and because each stick was left in the same conditioning room long enough to bring it to an approximate equilibrium moisture content with the temperature and humidity of 

Figure 8. Illustration of sorption isotherms measured under different conditions (Stamm and Loughborough 1935). 

The original figure caption reads, “Moisture content–

relative vapor pressure relationship for Sitka spruce under 

normal desorption and adsorption conditions and under 

oscillating vapor pressure desorption conditions at 25°C.”
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

9

the conditioning room, it may reasonably be assumed that 

the material was practically uniform in moisture content, 

i.e. that every stick had approximately the same moisture 

content.” Contrary to Menzel’s (1921) suggestion that variation in wood chemistry could explain variation in EMC, 

Loughborough assumed that there is a single “true moisture 

content” for all specimens. Consequently, he attributed any 

deviation from the mean to measurement error.

A further drawback to the documentation provided by 

Stamm and Loughborough (1935) is that the estimated error 

in RH was not reported. Dry- and wet-bulb thermometers 

were most likely calibrated regularly as part of standard 

laboratory procedure, using a thermometer standardized by 

the U.S. Bureau of Standards, as indicated in documentation 

by Loughborough (1922) prior to the work on Sitka spruce. 

However, the inherent measurement error in dry- and 

wet-bulb temperatures was likely not better than ±0.2 °F 

(±0.1 °C). Assuming this value, error in RH at room temperature is larger than ±1% RH (with higher error at lower 

temperatures but lower error at higher temperatures).

Finally, the thermodynamic analysis of Stamm and Loughborough (1935) is based on the assumption that the oscillating vapor pressure desorption method eliminates sorption 

hysteresis. Although they showed that the desorption curve 

under these conditions fell between the normal desorption 

and adsorption curves (Fig. 8), they did not measure any 

oscillating vapor pressure adsorption curves. To the best of 

our knowledge, no one has yet measured such a curve 

or shown that the method actually eliminates sorption 

hysteresis.

In summary, the Loughborough data set provides practical 

EMC values for Sitka spruce over a broad range of temperatures, but documentation of the data and methodology are 

incomplete: actual data points are not presented, the method 

of determining values at 100% RH is not described, EMC 

variability across specimens is undocumented, and measurement errors in EMC and RH are unknown.

2.5 Summary of Data Sources

Table 1 summarizes key information about each of the data 

sources described above. The first two sources collected 

EMC data on multiple species with limited number of specimens, whereas the last two sources focused on one species 

each with a large number of replicates. The first three 

studies were done at a single temperature (either ~212 °F 

(100 °C) or room temperature), whereas the last study covered a wide range of temperatures. From a scientific perspective, all the studies can be criticized for various problems in methodology and documentation. However, it must 

be kept in mind that the primary purpose of collecting EMC 

data at the Forest Products Laboratory during this era was to 

provide practical information related to drying lumber and 

controlling moisture content using the available resources. 

This theme is revisited in the next section.

3. Metamorphosis

This section traces the evolution of the ways in which data 

have been presented in FPL literature.

3.1 Table of EMC Values at Room Temperature

Technical Note F-13 (FPL 1919b), though anonymous, 

provides a data table that is clearly based on the work of 

Dunlap (1919). This table is reprinted here as Table 2. It 

lists EMC values “at ordinary temperatures” and RH levels 

between 20% and 95% (5% increments) for seven different woods as well as their average. This table is identical to 

table II in Dunlap (1919) except that it omits values at 100% 

RH, noting, “At 100 per cent humidity wood takes on moisture until saturated.” The statement regarding species nearly 

Table 1—Summary of FPL EMC data sources 

 H.E. McKenzie M.E. Dunlap C.A. Menzel W.K. Loughborough 

Wood species Spruce, longleaf pine, red 

oak, white oak, loblolly pine, 

tulip, hickory, blue gum, 

white ash 

 White oak, black walnut, 

Sitka spruce, yellow birch, 

ash, mahogany (African), 

longleaf pine 

 Red oak Sitka spruce 

Type of 

specimens 

 Blocks, 50 mm by 50 mm by 

13 mm 

 Groups of shavings, 

13 mm by 140 mm 

 Blocks, 50 mm by 

25 mm by 22 mm 

 Strips of flat-sawn lumber, 

3 mm thick 

Number of 

specimens 

Not stated 2 per species 46 25–40 at a given condition 

Temperature ~100 °C (~212 °F) 24–30 °C (75–86 °F) 27 °C (80 °F) 21–100 °C (70–212 °F) 

Sorption

direction 

Not stated Quasi-desorption (see 

description in Section 2.2)

 Adsorption and 

desorption 

 Oscillating vapor pressure 

desorption 

Concerns Poor documentation; 

questionable whether 

equilibrium was reached; 

measurement error unknown 

 Method of determining 

values at 100% RH not 

described; measurement 

error unknown 

 Imperfect RH 

control;

measurement error 

unknown

 Actual data points not 

documented; variability 

across specimens unknown; 

measurement error unknown 
General Technical Report FPL–GTR–229

10

duplicates Dunlap’s: “It will be noted that only slight differences are shown between the moisture-retaining qualities 

of the several woods tested, and the average values given, 

should, therefore, be roughly applicable for all woods.” The 

text briefly mentions other measurements: “Further tests on 

larger pieces of wood indicate that a variation of about 

1 per cent or less may be expected in the same species. 

These results accurately check the data given in the table between 20 and 70 per cent humidity. Above 70 percent values 

on larger blocks appear to be somewhat lower than given in 

the table.” This may be a reference to the work of Menzel 

(1921) on red oak blocks. However, as shown in Figure 6, 

Dunlap’s and Menzel’s measurements are in fairly good 

agreement, even though Dunlap’s measurements were taken 

in quasi-desorption from a wet initial condition, whereas 

Menzel’s measurements at 90% RH were taken in adsorption from a previous condition of 60% RH.

3.2 Table and Figure: EMC Values at Three 

Temperatures

Technical Note D-5 (FPL 1919a) provides EMC data at 

three temperatures, as reprinted here in Table 3. The values 

in this table agree with the curves in Figure 9a, taken from 

Koehler (1919). The legend makes it clear that the 70 °F 

(21 °C) curve is based on data from Dunlap, the 212 °F 

(100 °C) curve is based on data from McKenzie, and the 

141 °F (60.5 °C) curve is interpolated. (141 °F is midway 

between 70 °F and 212 °F. Presumably this value was 

rounded to 140 °F in Table 3.) Furthermore, the dashed parts 

of the curves indicate absence of data. This figure appears 

in nearly identical form in Koehler (1924) and Koehler and 

Thelen (1926).

Later bulletins from FPL include the same figure with slight 

modification, as shown in Figure 9b. This can be found in 

the Kiln Drying Handbook (Thelen 1923, 1929), The Air 

Seasoning of Wood (Mathewson 1930), Moisture Content of 

Wood in Dwellings (Peck 1932), the original edition of the 

Wood Handbook (FPL 1935), and numerous publications 

on wood aircraft (FPL 1928, 1941a, 1942, 1943, 1946). The 

legend has been removed, and there is no mention of the 

names Dunlap and McKenzie; furthermore, with the exception of the 1923 Kiln Drying Handbook, there is no mention 

that the 141 °F (60.5 °C) curve is interpolated or that the 

dashed parts indicate absence of data. This figure, without 

the legend (as in Fig. 9b), was reprinted in the Dry Kiln Operator’s Manual (Rasmussen 1961, Simpson 1991), as well 

as other texts (Siau 1995).

It can be seen in Figure 10a that the 70 °F (21 °C) values 

differ from Dunlap’s reported averages (which were 

Table 2—EMC table reprinted from Technical Note F-13 (FPL 1919b)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

11

reported as being taken at 75–86 °F (24–30 °C)); the 70 °F 

(21 °C) values in Table 3 and Figure 9a are generally lower 

than those of Dunlap except at 90% RH. Dunlap (1919) 

acquired measurements on seven species (see Section 2.2); 

however, the legend in Figure 9a indicates that the curve is 

based on the average of five species. Five out of the seven 

species that Dunlap studied also appear in a composite curve 

in Wood in Aircraft Construction (FPL 1919c) as shown in 

Figure 11. “African mahogany” may have been excluded 

because it was imported, and longleaf pine was likely excluded because it was not commonly used in aircraft construction. The composite curve in Figure 11 agrees well with 

the values in Table 3 (FPL 1919a). The averaging of five 

species rather than seven likely explains the discrepancy in 

Figure 10a.

Another instance of selecting five species can be found in 

Tiemann (1924), who notes that Dunlap’s EMC data for 

spruce and black walnut are “closely alike”; that white oak, 

yellow birch, and ash are “closely alike”; and that mahogany is “slightly higher” below 85% RH. Tiemann plots two 

curves based on Dunlap’s data with the noted similarities 

above: curve A for the average of white oak, yellow birch, 

and ash; and curve B for the average of spruce and black 

walnut. Oddly, Tiemann does not mention Dunlap’s data for 

longleaf pine but does mention data for shortleaf pine from 

Zeller (1920), even though Zeller had acquired data for both 

shortleaf and longleaf pine.

The 212 °F (100 °C) values in Table 3 and Figure 9a do 

not represent the true average of McKenzie’s data. There 

is no indication that certain species were excluded; Figure 

9a (Koehler 1919) states that the 212 °F (100 °C) curve is 

based on the average of nine species from McKenzie’s data. 

Figure 10b, however, shows that a curve fit to all the data 

points is considerably lower in EMC at high RH. The curve 

fit can be modified to agree with the values in Table 3 by 

excluding the data for hickory, blue gum, and white ash at 

values above 33% RH (shown as the “modified fit” in Figure 10b). Again it is not clear why these data should have 

been excluded. There were no suggestions in McKenzie’s 

report that certain species ought to be excluded.

The issue of interpolation demands further attention. 

Koehler and Thelen (1926) appear to be the first authors to 

provide a qualifying statement regarding interpolation of 

Table 3—EMC table reprinted from Technical Note D-5 (FPL 1919a)

Figure 9. EMC–RH curves from (a) Koehler (1919) and 

(b) Simpson (1991).
General Technical Report FPL–GTR–229

12

EMC values at different temperatures: “The curve at 141°F. 

is interpolated halfway between the other two curves. Recent observations show that this curve should be somewhat 

lower, although its exact location has not been determined. 

With this fact in mind, curves for other temperatures may 

be interpolated fairly accurately.” The mention of “recent 

observations” may be a reference to the (then unpublished) 

measurements of Loughborough (Section 2.4). Two observations lend support to this hypothesis. First, a revision 

(FPL 1941b) of Koehler’s (1919) article did not include 

Figure 9a, but replaced it with the figure based on the work 

of Loughborough (see Fig. 7). Second, the same figure swap 

0

5

10

15

20

25

30

35

40

20 40 60 80 100

E

quillibrium

m

oisture content (%)

Relative humidity (%)

Dunlap (1919) reported average

FPL (1919a), 70 °F

(a)

2

4

6

8

10

12

14

16

20 40 60 80 100

E

quilibrium

m

oisture content (%)

Relative humidity (%)

Spruce I

Spruce II

Longleaf pine

Loblolly pine

Red oak

White oak

Tulip

Hickory

Blue gum

White ash

FPL (1919a), 212 °F

Fit to all data

Modified fit

(b)

0

0

0

Figure 10. Comparison of EMC values tabulated in Technical Note D-5 (FPL 1919a) with (a) average values reported by 

Dunlap (1919) and (b) values reported by McKenzie (n.d.). The “modified fit” in (b) excludes hickory, blue gum, and white 

ash data above 33% RH.

Figure 11. EMC data from Wood in Aircraft Construction (FPL 1919c).
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

13

can be found by comparing Wood-Liquid Relations (Hawley 

1931) with a marked-up draft manuscript entitled WoodFluids Relations4 [sic]. The latter includes Figure 9a, but 

the curve labeled 141 °F (60.5 °C) is crossed out with red 

pencil. The published version (Hawley 1931) uses the figure 

based on the work of Loughborough (see Fig. 7).

In summary, the ways in which Dunlap’s and McKenzie’s 

data were presented in the literature over time display an 

evolution characterized by selective averaging, gradual loss 

of citation of the data sources, and loss of explicit distinction between curves based on measurement and those based 

on extrapolation or interpolation.

3.3 Preliminary Figure and Table with 

Broad Temperature Range

Prior to completion of Loughborough’s data set, EMC values appeared in preliminary form, as reprinted in Figure 12, 

taken from Seasoning, Handling, and Care of Lumber [Distributor’s edition] (U.S. Department of Commerce 1928a). 

This figure appears in the context of a discussion of controlling moisture content of lumber in heated storage. Regarding this chart, the report says, “...its chief value is that it is 

the only one thus far known which gives adequate information regarding the equilibrium moisture content of wood at 

4See Bateman et al. (n.d.). On page one of the document, a footnote states 

the following: “This article is a contribution of the Liquid-Wood Relationship Committee of the Forest Products Laboratory. The committee consists 

of L.F. Hawley (Chairman), Bateman, Brown, Dunlap, Koehler, Loughborough, Stamm, Tiemann. All members of the committee contributed more or 

less to the paper but the work of assembling and codifying the information 

and the preparation was done by a sub-committee whose names appear as 

authors.”

various stages of air temperature and humidity.” The figure 

caption in the same report reads, “First publication, by 

courtesy of Forest Products Laboratory, of a chart still under 

revision and correction, but showing tentatively the equilibrium moisture content of wood over a wide range of air temperatures and humidities. The vertical lines show dry-bulb 

air temperatures according to top and bottom scales; the 

horizontal lines show relative percentages of air humidity 

according to right-hand scale; the curved lines show depressions of wet bulb below dry bulb according to the scale on 

the face of the chart; and the lines slanting upward from the 

left side of the chart are lines of moisture content in wood, 

read on the left-hand scale.”

The “Fabricators’ edition” of the same report (U.S. Department of Commerce 1928b) includes further detail on the 

preparation of Figure 12. It first discusses the earlier chart 

(see Figure 9a), which has EMC curves at 70 °F (21 °C) and 

212 °F (100 °C) and notes that the 141°F (60.5 °C) curve 

was produced by interpolation. It then states, “The chart 

here shown ... has been developed from the original chart, 

partly by interpolation and partly by additional measurement tests at other temperatures. It has been withheld from 

publication pending refinement and revision by further measurement tests but is doubtless already amply reliable for 

the purpose for which it has been used here, and its release 

to this use by the laboratory is deserving of special recognition.” Unlike the previous chart, however, Figure 12 does 

not use dashed curves to indicate portions that have been 

interpolated. Certainly portions were also extrapolated; nowhere in the data sources do we find measurements down to 

0 °F (–18 °C).

Figure 12. EMC–humidity–temperature chart (U.S. Department of Commerce 1928a).
General Technical Report FPL–GTR–229

14

As the chart is difficult to read accurately, the report also 

includes a table of EMCs over a range of dry-bulb temperature and wet-bulb depression (the difference between dryand wet-bulb temperatures), reprinted here in Table 4. The 

corresponding RH range calculated from these values varies 

from 16% to 74% at 50 °F (10 °C), from 24% to 82% at 

75 °F (24 °C), and from 30% to 86% at 100 °F (38 °C).

Graphs of the same form as Figure 12 have subsequently 

been published, presumably based on the completed data 

set, and no longer considered to be under revision and correction (Voorhies and Loughborough 1943; Smith 1947, 

1963; Denig et al. 2000).

3.4 EMC and RH Figures and Tables from 

30 °F to 210 °F and Beyond

The original presentation of Loughborough’s data was in the 

form of Figure 7, with a temperature range of 70 °F (21 °C) 

to 212 °F (100 °C). As stated above, the original discrete 

data points were never presented in graphic or tabular form. 

The original figure (Fig. 7) was printed in several works in 

the 1930s to early 1950s (Hawley 1931; Loughborough and 

Rietz 1932; Stamm and Loughborough 1935; FPL 1941b, 

1942, 1944, 1951).

Several variations on Figures 7 and 12 have also appeared in 

the literature. We include a sample of these here. Figure 13 

shows relative humidity versus dry-bulb temperature with 

curves of constant EMC and of constant vapor pressure. 

Figures of this type have been widely published (Peck 1932; 

FPL 1935, 1940, 1944, 1951, 1955; U.S. Army Air Forces 

1944; Denig et al. 2000). Figure 14 shows relative humidity 

versus vapor pressure with curves of constant EMC and of 

constant temperature (Loughborough 1942). Both of these 

figures are essentially transformations of Figure 7 with different axes and a constricted temperature range. Finally, 

Figure 15 is a plot of wet-bulb temperature versus dry-bulb 

temperature with curves of constant EMC (Rietz 1931; 

Smith 1947, 1963).

During the 1940s, a table appeared giving EMC and RH as 

a function of dry-bulb temperature and wet-bulb depression, 

presumably based on Loughborough’s measurements. This 

table (see Table 5) has been widely reprinted in FPL publications (FPL 1944, 1946, 1951, 1952, 1955, 1999a; Johnson 

and Rasmussen 1946; Smith 1947, 1963; Rasmussen 1961; 

Simpson 1991; Denig et al. 2000).

Several important differences between the values in Figure 7 

and Table 5 are evident:

•	 The original figure displays curves for temperatures every 

10 °F from 70 °F to 200 °F plus 212 °F. In contrast, the 

table begins at 30 °F, increases in 5 °F steps to 130 °F, 

and then increases in 10 °F steps to 210 °F.

•	 The original figure displays solid curves (indicating that 

they are not extrapolated) up to 100% RH at temperatures 

of 70 °F, 80 °F, and 90 °F. At all other temperatures, the 

curves are dashed above 90% RH, indicating that they 

are extrapolated. The table lists EMC values at wet-bulb 

depression as low as 2 °F, which corresponds to relative 

humidity values of 78% RH at 30 °F dry-bulb, 90% RH 

at 70 °F dry-bulb, and 96% RH at 210 °F dry-bulb.

The temperature range of the original data set is unclear. 

We entertain two hypotheses. The first is that the original 

temperature range extended down to 30 °F (–1 °C), but temperatures below 70 °F (21 °C) were excluded from Figure 7 

because the curves at such temperatures would be difficult 

to distinguish. This explanation is consistent with the fact 

that a preliminary table (U.S. Department of Commerce 

1928a), reprinted in Table 4, gave EMC values down to 

50 °F (10 °C). An alternative hypothesis is that 70 °F (21 

°C) is the lower limit of the original temperature range and 

that additional measurements were taken after 1935 to exTable 4—Variation of EMC with dry-bulb temperature and wet-bulb depression 

(reprinted from U.S. Department of Commerce 1928a)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

15

the basis of this figure, we propose that the original data 

were measured at increments of 10% RH. This is consistent 

with the fact that Figure 7 shows 90% RH as the upper limit 

for the solid curves (measured values) at temperatures of 

100 °F (38 °C) and higher.

We further suggest that Table 5 was generated by interpolation and extrapolation, based on the measured data. This 

argument is consistent with the fact that the form of Table 5 

predates the EMC data, which is evident from a comparison 

of two different versions of an FPL Technical Note, both 

bearing the number 156: the Table of Relative Humidity 

and Equilibrium Moisture Content for Dry- and Wet-Bulb 

Hygrometer (FPL 1952) is a revision of the Humidity Table 

for Wet and Dry Bulb Hygrometer (FPL 1921). The latter is 

reprinted in Table 6; it gives only RH values as a function of 

dry-bulb temperature and wet-bulb depression. The layout is 

similar to Table 5, but with slight differences in temperature 

range and increment. This RH table was reprinted widely 

in FPL literature during the late 1910s to early 1940s, with 

only slight changes in temperature range and increment 

(Koehler 1919, 1924; Koehler and Thelen 1926; Thelen 

1923, 1929; Mathewson 1930; FPL 1941b, 1942). Given the 

practical use of this table for kiln operation, we suggest that 

it would have been consistent with FPL trends noted previously to supplement the tabulated RH values with calculated 

EMC values.

Figure 13. Relative humidity versus dry-bulb temperature, with curves of constant vapor pressure and of constant EMC (FPL 

1955).

tend the temperature range down to 30 °F (–1 °C). Under 

this hypothesis, the EMC values at temperatures below 70 

°F (21 °C) given in 1928 would have been extrapolated. It 

is not possible to determine which hypothesis is correct because documentation is lacking.

It has previously been implied (Simpson 1971, 1973, 1980) 

that Table 5 represents actual measured data points. Although it is possible that the table contains the actual data 

points, we find no solid support that these were actually 

measured values; the evidence is inconclusive. On one hand, 

the form of the table listing dry-bulb temperature and wetbulb depression corresponds with the method in which the 

data were collected: dry- and wet-bulb temperatures were 

controlled with thermostats (Section 2.4.2). On the other 

hand, the table does not appear in publications until over a 

decade after publication of graphs. As discussed above, the 

Forest Products Laboratory commonly published smoothed 

curves and tables for practical application, using interpolation and extrapolation from a limited set of measurements, 

rather than publishing the actual measurements. Given this 

atmosphere, it seems unlikely that a total of 826 values 

were actually measured (Table 5) when a smaller number 

of measurements would have provided a sufficient basis for 

interpolation.

A rare instance in which discrete data points were published 

is shown in Figure 8 (Stamm and Loughborough 1935). On 
General Technical Report FPL–GTR–229

16

Additional evidence consistent with the argument that the 

values in Table 5 were calculated rather than measured 

can be found by tracing the source of the RH tables. Of 

all the FPL publications that print an RH table, Koehler 

(1924) appears to be the only one to cite his source, which 

is the humidity diagram of Tiemann (1912). This diagram 

is reprinted in Figure 16. RH is the y-axis (0–100%), and 

dry-bulb temperature is the x-axis (–30 to 220 °F). Convex 

curves are plotted for constant values of wet-bulb depression (with corrections for barometric pressure), and concave 

curves are plotted for constant water vapor pressure, with 

labels indicating water vapor density (absolute humidity) at 

the temperature of saturation. Tiemann indicates that the relationship between RH, dry-bulb temperature, and wet-bulb 

temperature is based on Ferrel’s formula (see Marvin 1900), 

but has been extended to higher temperatures. Saturation 

vapor pressures are based on the U.S. Weather Bureau’s 

Psychrometric Tables (Marvin 1900) and the Smithsonian 

Meteorological Tables (Smithsonian Institution 1907). Tiemann states that although Ferrel’s formula is strictly applicable only below 140 °F (60 °C) (the parameters were deduced from experiments conducted only at temperatures below this), he had reason to extend it to higher temperatures: 

“Experiments made by the author indicate, however, that the 

equation holds with reasonable exactness for temperature 

up to the boiling point, and the curves have therefore been 

extended to 220 °[F].” The humidity diagram is reprinted in 

several publications, with slight modification in some cases 

(Tiemann 1917, FPL 1919c, Koehler and Thelen 1926, U.S. 

Army Air Forces 1944, Smith 1963).

The similarity between Figures 12 and 16 is striking. 

Figure 16 was developed as a tool for use in lumber 

drying. The figure allows conversion of psychrometric 

Figure 14. Relative humidity versus vapor pressure, with lines of constant dry-bulb temperature and curves of constant 

EMC (Loughborough 1942).
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

17

Figure 15. EMC as a function of wet- and dry-bulb temperatures (Rietz 1931; Smith 1947, 1963).
General Technical Report FPL–GTR–229

18

parameters that were commonly measured (wet- and drybulb temperatures) to other useful parameters (RH, dewpoint temperature, and water vapor density). Figure 12 

adopts the same form as Figure 16 but inserts wood EMC 

curves while removing the water vapor density curves. 

Later versions of both Figures 12 and 16 are given by Smith 

(1963). This simple graphical transformation was likely paralleled in tabular form, as suggested above. It would be 

consistent with previous trends at FPL, once a full EMC 

data set was available, to take an existing RH table 

(Table 6) and supplement it with interpolated/extrapolated 

EMC values (Table 5).

During the 1970s a further transformation occurred in EMC 

tables: wet-bulb depression disappeared from the table and 

was replaced by relative humidity. Table 7 shows the table 

as printed in the 1974 edition of the Wood Handbook (FPL 

1974). It should be noted that the temperature values range 

from 30 °F to 210 °F in increments of 10 °F and that the 

relative humidity values go from 5% to 95% in increments 

of 5% RH plus 98% RH.

It is not clear how the values in the Table 7 were derived. 

These values may have been estimated by interpolation/

extrapolation using the previously tabulated values, or they 

may have been calculated from an equation whose parameters were based on least-squares fitting to the previously 

tabulated values.

Simpson (1971, 1973) used the EMC values tabulated in the 

1955 Wood Handbook (FPL 1955) to determine parameters 

for a thermodynamic model developed by Hailwood and 

Horrobin (1946). The model partitions bound water into 

“a solid solution of water in the polymer” and “hydrates 

between water and definite units of the polymer molecule.” 

An arbitrary number of hydrates can be added; equations 

with parameters fit to the FPL EMC values have been published with one hydrate (Simpson 1971) and two hydrates 

(Simpson 1973). We refer to the latter as the two-hydrate 

HH equation:

(2)

where M is equilibrium moisture content (percentage), h is 

relative humidity (decimal), and the parameters Mp, K, K1, 

and K2 have a particular physical significance in the model 

derivation of Hailwood and Horrobin (1946). To account for 

the temperature dependence of the EMC values, Simpson 

(1973) made each parameter a second-order polynomial in 

temperature T (in °F) as follows:

 Mp = 330 + 0.452T + 0.00415T 2

K = 0.791 + 0.000463T - 0.000000844T2

K1 = 6.17 + 0.000313T - 0.0000926T2

K2 = 1.65 + 0.0202T - 0.0000934T2

By comparing the values tabulated in the 1974 Wood Handbook with those generated from the equations given by 

Simpson (1971, 1973), we find differences of up to 2.7% 

MC for the one-hydrate equation and up to 0.5% MC for the 

two-hydrate equation. These differences make it clear that 

the 1974 Wood Handbook tabulated values were not generated directly from either of Simpson’s HH equations.

In the 1987 Wood Handbook (FPL 1987), a two-hydrate 

HH equation appears.5 The parameters in this equation are 

very similar (but not identical) to those in Equation (3). The 

temperature coefficients of the K1 and K2 parameters given 

in the 1987 Wood Handbook (Eq. (4)) differ from those published by Simpson (1973) (Eq. (3)).

 K1 = 6.34 + 0.000775T - 0.0000935T2

K2 = 1.09 + 0.0284T - 0.0000904T2

The reason for this difference is unknown. However, it just 

happens that the EMC values generated using these parameters are in excellent agreement with the tabulated EMC values in the 1974 and 1987 editions of the Wood Handbook, 

the maximum difference being less than 0.1% MC (for temperatures from 30 °F to 210 °F). In order to determine which 

set of K1 and K2 parameters is correct, we compared EMC 

values predicted by both sets of parameters with values in 

the 1955 Wood Handbook. We find that the parameters in 

Equation (3) (Simpson 1973) fit the data better than those in 

Equation (4) (FPL 1987): the RMSE and the maximum absolute error are lower when the parameters in Equation (3) 

are used. The differences, though, are small; on average the 

calculated EMC values differ by less than 0.2% MC. It is 

unclear why the K1 and K2 parameters were changed.

An additional transformation in the 1987 Wood Handbook

is that the tabulated EMC values extend to 270 °F (132 °C) 

(see Table 8). This extrapolation may have been based on 

the work of Simpson and Rosen (1981), which showed that 

the one-hydrate HH equation using the parameters of Simpson (1971) when extrapolated to 300 °F (149 °C) compared 

reasonably well with measured EMC values at these higher 

temperatures from the broader literature. However, the tabulated values in the 1987 Wood Handbook for temperatures 

from 220 °F (104 °C) to 270 °F (132 °C) appear to be generated from the two-hydrate HH equation (with K1 and K2

parameters from Eq. (4)) rather than the one-hydrate 

HH equation. According to our analysis, the values in 

Table 8 and those calculated from the two-hydrate HH equation differ by not more than 0.2% MC; in contrast, the onehydrate HH equation gives values that differ by up to 1.6% 

MC from those in the table. The EMC values predicted by 

the two equations diverge with increasing temperature. For 

example, EMC values at 30% RH at 220 °F (104 °C) are 

3.0% and 2.9% for the one-hydrate and two-hydrate HH 

5The notation in the 1987 Wood Handbook is incorrect: relative humidity 

should have been given as a decimal (0 ≤ h ≤ 1) rather than as a percentage. 

This was corrected in the 1999 Wood Handbook.

M = 1,800

Mp

Kh

1 – Kh

+

K1Kh + 2K1K2K2h2

1 + K1Kh + K1K2K2h2

(3)

(4)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

19

Figure 16. Humidity diagram from Tiemann (1912).
General Technical Report FPL–GTR–229

20Table 5—Variation of relative humidity and EMC with dry-bulb temperature and wet-bulb depression (reprinted from FPL 1952)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

21

Table 6—Variation of relative humidity with dry-bulb temperature and wet-bulb depression (reprinted from FPL 1921)
General Technical Report FPL–GTR–229

22Table 7—EMC table reprinted from the 1974 Wood Handbook (FPL 1974)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

23

equations, respectively; at 270 °F (132 °C), the values are 

2.0% and 0.4%, respectively.

We also note that the 1974 and 1987 editions of the Wood 

Handbook extrapolate EMC values to 98% RH (see also 

ASTM 2010). The 1999 Wood Handbook (FPL 1999b) restricts EMC values to 95% RH (see Table 9). Although the 

RH range of the original data is not entirely clear, the 

earliest publications indicate that at most temperatures, 

data were measured at RH levels no higher than 90% (see 

Section 2.4).

3.5 Summary of Metamorphosis

The presentation of EMC data in FPL literature evolved as 

new measurements became available and as practical needs 

changed. We have highlighted three patterns in this metamorphosis: (1) reduction of data by selective averaging; 

(2) reuse of figures accompanied by loss of citation of data 

sources; and (3) recycling of data into new tables and figures, with loss of distinction between values based on measured data and those based on extrapolation or interpolation.

4. Are the Data Valid for Scientific 

Purposes?

Although this investigative report of the genesis and metamorphosis of the FPL EMC data set is not intended to be 

a review of the subject of wood–water relationships, it is 

clear, even from this limited perspective, that the interaction of water with biologically produced materials, such 

as wood, is a very complex and difficult area of study. 

What is required to advance the knowledge of wood–water 

relationships are more creative experiments that expand 

the response space, that is, tracking moisture content and 

dimensional changes (and perhaps other pertinent wood 

properties) as the wood samples are moved about in various 

temperature and relative humidity state space points. Corresponding mathematical models are needed that not only 

duplicate the measured phenomena reasonably well but also 

provide explanatory power.

To this end, the FPL-generated EMC data set will be of limited value with respect to uncovering a deeper understanding 

of wood–water relationships. Moreover, because of 1) the 

lack of proper documentation of methodology and procedures, 2) the unsolvable problem of knowing which values 

are determined from direct observations and which are interpolated, and 3) the absence of definitive measurement error 

analysis, the FPL data set lacks the certification demanded 

of the scientific method; i.e., that truth is conditioned by the 

fundamental idea that sufficient information must be supplied with the results in order for others to duplicate them. 

In this sense, the FPL data set is strictly unreliable with 

regard to testing any hypothesis concerning wood–water 

relationships. That does not take away the value of the data 

as being a first approximation for average values in the 

temperature, moisture content, relative humidity, pathway 

description state space. The data set, unfortunately, must be 

regarded as unreliable when it comes to being used in scientific studies.

5. Practical Applicability of the Data

EMC data are used for a range of practical applications, 

some of which include lumber drying, conditioning of wood 

specimens to desired moisture content prior to testing physical or mechanical properties, and modeling of moisture 

content of wood members in buildings. The degree of error 

in EMC values that can be tolerated may differ depending 

on the application. Variability in EMC values may arise 

from several sources such as wood chemistry, temperature, 

and specimen history—including sorption hysteresis as well 

as irreversible effects such as exposure to high temperature, 

chemical treatment, or radiation. In the case of lumber drying, the moisture content history of the material is known; 

it is taken from a high initial MC to a lower MC. In other 

cases, the moisture content history may not be known, and 

sorption hysteresis cannot be accounted for.

It is important here to distinguish experimental error in 

EMC determination from inherent variability in EMC across 

specimens of a given type of material. This distinction was 

previously discussed in Section 2.4.3. Apart from a detailed 

analysis of experimental error, it is not possible to say anything meaningful about inherent variability. Measurement 

error for gravimetric moisture content determination depends on the balance weighing accuracy and specimen size; 

on this basis an estimated error of approximately ±0.1% MC 

is easily achievable. However, additional sources of error in 

determining EMC may include the criterion used for defining equilibrium, fluctuations in specimen mass near equilibrium resulting from temperature or RH fluctuations, and the 

method used to determine specimen dry mass. Sources of 

error for the oven drying method include non-zero RH in the 

oven and possible evaporation of volatile wood constituents 

other than water. These sources of error are discussed in 

detail by Skaar (1988). Error in RH determination may vary 

widely depending on the means used to control RH, such 

as saturated salt solutions or mechanically controlled environmental chambers, and the accuracy of the sensor used to 

measure humidity (as dew point temperature, wet-bulb temperature, or relative humidity).

In this section, we review claims in the FPL literature regarding the applicability of the FPL EMC data set to different wood species, review perspectives on sorption hysteresis 

in the FPL literature, examine the ability of the data set to 

represent the best fit to literature data for one species group, 

and compare the data set with literature values for various 

species.

5.1 Claims Regarding Wood Species 

in FPL Literature

Speculation that all wood species have the same EMC–RH 

relationship started with Dunlap (1919), as mentioned 
General Technical Report FPL–GTR–229

24Table 8—EMC table reprinted from the 1987 Wood Handbook (FPL 1987)
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

25

previously (Section 2.2.2): “It is interesting to note that 

there is comparatively little variation in the values obtained 

for the individual species tested and in a general way it is 

thought that the average curves would apply practically to 

any wood.”

Publications in the 1920s generally included figures similar to Figure 9a and echoed Dunlap’s speculation. Koehler 

(1924), for example, appealed to wood chemistry for an 

explanation: “It is interesting to note that this relation is approximately the same for all woods. This might be expected 

when one considers that the substance of which wood is 

made is practically the same for all species.” On the other 

hand, might it not be expected that differences in wood 

chemistry among species could lead to variation in hygroscopicity? Later investigations would make this apparent, 

as discussed below.

The tendency to speculate regarding species continued 

throughout FPL history. We note a few further examples.

•	 Koehler and Thelen (1926): “Although these curves are 

based on only a very limited number of species of wood, 

the tests showed that the different species were remarkably similar their moisture-humidity relations. 

It is believed that these curves are reasonably accurate for 

almost all woods.”

•	 Loughborough and Rietz (1932): “Other species might 

show some slight variations, but the general shape of 

the curves would be the same for all species. For practical engineering purposes these values can be considered 

about the same for all species of wood.”

•	 FPL (1941b): “These curves were constructed from data 

obtained on Sitka spruce, but are in general applicable to 

all species.”

•	 Wood Handbook (FPL 1955): “Although different species exhibit some differences in their reactions to relative 

humidity, for practical purposes [Table 5] applies to the 

wood of any species.” Later editions of the Wood Handbook essentially made the same claim.

Nowhere in the FPL literature do we find a suitable definition of general or practical applicability. Non-quantitative, 

blanket statements regarding accuracy of the data for any 

species, in conjunction with tabulated EMC values at a high 

level of precision (to 0.1% MC in Table 5), could be misleading. We discuss this matter further in following sections.

Table 9—EMC table reprinted from the 1999 Wood Handbook (FPL 1999b)
General Technical Report FPL–GTR–229

26

5.2 Perspectives on Sorption Hysteresis in 

FPL Literature

Koehler and Thelen (1926) present the concept of hysteresis 

for the first time in FPL literature (to the best of our knowledge) in regard to Figure 9a:

“The upper curve in the figure shows the moisture 

content which wood will attain when subjected to 

different humidities at ordinary room temperature 

(70°F.). At a humidity of 68 per cent, for instance, 

wood, if wet, will dry out to approximately 13 per 

cent; or if it is very dry, it will absorb moisture to 

about 13 per cent of its weight. In fact, the dry wood 

will not attain quite so high a moisture content as 

13 per cent, and the green wood will not become quite 

so dry. The curve represents the average moisture 

content of wet wood losing moisture and dry wood 

gaining moisture at certain humidities.”

Hawley (1931) briefly comments, “like pure cellulosic fibers, wood has a hysteresis effect, with a difference in the 

equilibrium values when absorbing and when giving off 

moisture,” citing literature from outside FPL.

The most complete discussion of hysteresis in FPL literature 

is given by Stamm and Loughborough (1935), as discussed 

in Section 2.4.2. Figure 8 compares data for the normal 

adsorption and desorption curves and the intermediate 

“oscillating vapor pressure-desorption curve.” Stamm and 

Loughborough (1935) also state, “All the available drying 

data obtained under industrial control conditions on other 

softwood species also gave curves which fell within this 

hysteresis loop.”

Subsequent FPL literature tended to remain silent regarding 

hysteresis with a few exceptions (e.g., Simpson 1973, Simpson and Rosen 1981). For example, the Wood Handbook

did not discuss sorption hysteresis in its 1935, 1940, 1955, 

1974, and 1987 editions; hysteresis was first mentioned in 

the 1999 edition. From a practical perspective, hysteresis 

is not terribly important in wood drying; hence no need to 

mention it. The 1999 Wood Handbook (FPL 1999b) takes a 

more general view of the practical utility of data obtained 

under conditions of oscillating vapor pressure desorption, 

saying the data represent “a suitable and practical compromise for use when the direction of sorption is not always 

known.” In the next section, we investigate this claim using 

literature data for the southern yellow pine (SYP) species 

group.

5.3 Comparison with Literature Data for 

Southern Yellow Pine

Here we attempt to quantify the notion of practical applicability based on statistical analysis of literature EMC data. 

We selected data for species within the SYP group because 

numerous measurements have been published over many 

years. Figure 17 plots data from a variety of sources: Dunlap 

(1919), Nearn (1955), Higgins (1957), Choong (1969), Lee 

and Biblis (1976), Richards et al. (1992), Cao and Kamdem 

(2004), and Zelinka and Glass (2010). All of the measurements were taken between temperatures of 70 °F (21 °C) 

and 90 °F (32 °C). The pooled data set includes adsorption 

and desorption measurements. In addition to sorption hysteresis, variability in EMC values could be expected because 

the publications from which the data were selected span a 

broad range in time and geographic location. An analysis of 

experimental error in each of the data sources is beyond the 

scope of this report. Figure 17 shows three-parameter parabolic fits for desorption alone (63 data points), for adsorption alone (62 data points), and for all data points together 

(“global fit”). The fitting equations are of the same form as 

Equation (1). Additionally, the curve of Simpson (1973) representing the FPL data at 80 °F (27 °C) is shown in Figure 

17; this curve does in fact fit the measured data very well. 

The curve for the four-parameter equation from Simpson 

(1973) is somewhat lower than the best fit curve for the 

3-parameter equation at low RH levels, while the two curves 

are nearly identical above 40% RH.

Table 10 gives statistics related to the three-parameter fits 

shown in Figure 17. The fit to all data points has a rootmean-square error of 1.7% MC. This error decreases to approximately 1% MC when the adsorption data and desorption data are fit separately. The RMSE for the FPL curve and 

all the data points (not shown in the table) is 1.8% MC, only 

slightly higher than that for the three-parameter best fit.

This comparison highlights several important points. First, 

it lends support to the previously mentioned claim that the 

data represent “a suitable and practical compromise for use 

when the direction of sorption is not always known” (FPL 

1999). The FPL curve falls nearly in the middle of all the 

SYP data points. Second, it illustrates the obvious point 

5

10

15

20

25

30

35

0 20 40 60 80 100

Equilibrium

m

oisture content (%)

Relative humidity (%)

Desorption

Adsorption

Desorption fit

Adsorption fit

Global fit

FPL curve

0

Figure 17. EMC data from the literature for the southern yellow pine species group near room temperature with curve 

fits (see text). FPL curve is from Simpson (1973).
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

27

that sorption hysteresis has a substantial effect. Third, it 

shows that in addition to sorption hysteresis, there are other 

sources of variability within a species group. For this reason, the FPL data do a good job predicting the average EMC 

of the group but do not reliably predict the EMC value of 

every specimen. Finally, use of the FPL EMC values at the 

reported level of precision (to 0.1% MC in Table 5) would 

be misguided. Although this level of precision is useful for 

showing trends in EMC with RH and temperature, the assumption that this level of precision is meaningful not only 

neglects the effect of sorption hysteresis but also far exceeds 

the precision required for most practical purposes.

5.4 Comparison with Literature Data for 

Different Wood Species

Research conducted outside FPL in the 1950s to 1970s 

looked at differences in EMC between wood species near 

room temperature (Nearn 1955; Higgins 1957; Spalt 1958; 

Hedlin 1967; Wangaard and Granados 1967; Choong and 

Manwiller 1976). These studies indicate that EMC differences between species generally are minor at low and moderate levels of relative humidity but become considerable at 

high RH levels. Species with high extractive content generally show lower EMC at high RH levels than species with 

low extractive content.

As a simple illustrative comparison of the FPL curve to literature EMC data of different species, we selected data from 

one source—Spalt (1958)—because it includes 16 different 

species. Spalt measured 6 or 7 data points each for adsorption and desorption (12–14 data points for each species) 

at 32 °C (90 °F). He reported curve fit parameters (using a 

three-parameter parabolic equation, similar form to that given above in Eq. (1)) for the adsorption and desorption data 

for each species. Figure 18 shows the 32 curves (adsorption 

and desorption curves for each of 16 species) together with 

the FPL curve at the same temperature. Qualitatively, the 

FPL curve appears to be a suitable average. Table 11 lists 

the differences between Spalt’s adsorption and desorption 

curves and the FPL curve for each species, at RH levels of 

65%, 80%, and 95%. In general, differences increase with 

increasing relative humidity. At 65% RH, 

the values from FPL and from Spalt differ by as much as 

3.2% MC; at 80%, by 3.4% MC; and at 95% RH, by 5.9% 

MC. Table 11 also shows that the RMSE for the FPL curve 

and all 16 species increases with increasing RH in adsorption, desorption, and the average of the two.

In conclusion, the FPL EMC data provide a suitable average 

for many species for practical purposes, but past FPL literature has overemphasized the similarities and minimized the 

differences in EMC values between species. Research showing between-species differences in EMC at high RH levels 

has not been acknowledged in the Wood Handbook; we recommend that future editions of the Wood Handbook address 

this issue. We also stress that the precision with which EMC 

values have been given in past FPL literature such as 

Table 5 (to 0.1% MC) far exceeds the precision required 

for most practical purposes.

6. Mathematical Formulations for 

Practical Use

Given the magnitude of the variability in EMC values resulting from sorption hysteresis and from differences in 

wood chemistry, we question the need for an equation as 

complicated and precise as Equation (2). For practical 

purposes, might a simpler empirical equation with fewer 

parameters be sufficient?

Table 10—Statistics related to fitting literature EMC data for 

southern yellow pine 

 Error (% MC) 

Data set Measurements 

Root-meansquare error Mean absolute 

error 

Maximum 

absolute error 

All data 125 1.7 1.4 4.0 

Desorption 63 1.1 0.9 2.4 

Adsorption 62 1.2 1.0 3.4 

5

10

15

20

25

30

0 20 40 60 80 100

E

quilibrium

m

oisture content (%)

Relative humidity (%)

0

Figure 18. EMC adsorption and desorption curves for 16 

species from Spalt (1958), shown in gray, and FPL data 

shown as heavy black curve (Simpson 1973).
General Technical Report FPL–GTR–229

28

As discussed above (Section 3.4), the two-hydrate HH 

equation was fit to the FPL EMC values by Simpson (1973). 

When temperature dependence is included, the model 

effectively changes from one with four fitting parameters to 

one with 12 fitting parameters, and hereafter we refer to this 

model as the HH12 model. While the HH12 model fits the 

data well, it is cumbersome to use and nearly impossible to 

invert.

If the HH12 model parameters indeed had physical significance relevant to the wood–water system, then giving 

preference to this model might be justified. The connection 

between model parameters and physical reality, however, is 

weak. One thermodynamic property that is often incorporated into models for vapor sorption is the differential heat 

of adsorption. Simpson (1980) showed that many different 

sorption models (including the one-hydrate and two-hydrate 

HH models) predict values for the differential heat of adsorption that drastically differ from experimental values. 

Furthermore, two other models that have been applied to 

wood—the Dent (1977) isotherm and the GuggeheimAnderson-deBoer (GAB) isotherm (Anderson 1946)—are 

mathematically equivalent to the one-hydrate HH model; 

they can all be reduced to the form h/m = Ah2 + Bh + C, 

where h is relative humidity and m is EMC, the same generic parabolic model used previously (Eq. (1)). However, 

these three isotherms were derived from different starting 

points and ascribe different physical meanings to the parameters. Although the Dent, GAB, and HH isotherms provide 

excellent fits to wood EMC data, the physical pictures associated with these models cannot all be true, and therefore 

these models should not be used for purposes other than 

curve fitting. Because the HH12 model does not give 

additional physical insight, it is worthwhile examining 

whether a simpler equation is adequate for curve fitting.

Avramidis (1989) previously used a subset of the FPL EMC 

data to evaluate four sorption models that express moisture 

content as a function of both relative humidity and 

temperature. These models had two to four fitting parameters. Curve fitting was limited to EMC values at 70, 95, 110, 

125, and 160 °F (21, 35, 43, 52, and 71 °C) from the 1955 

Wood Handbook (FPL 1955). Avramidis found that the model of Zuritz et al. (1979) gave the best fit to EMC data in this 

temperature range based on RMSE. This model is actually 

a modification of the equation of Henderson (1952) with 

4 parameters. We refer to the model as the 4-parameter Henderson (or H4) model hereafter to highlight the similarity.

Here we performed a curve-fitting analysis using a wider 

range of models over the complete range of temperatures 

given in Table 5. We considered models that have from two 

to six fitting parameters, as well as the HH12 model for 

reference. Lewicki (2009) recently reviewed 31 sorption 

models that have been applied to solid food substances and 

grouped them into those that were theoretically derived, 

those that were empirical, and those that were hybrids (or 

semi-empirical). From this list of 31, we chose six models 

to compare to the HH12 Equation (Table 12). We gave 

preference to simple, empirical models that are invertible, although we did include a theoretically derived model 

(GAB); however, we emphasize that in these cases, the 

model parameters should not be interpreted as having physical meaning.

Table 11—Comparison of FPL EMC data with literature values for various species 

Difference in EMC (% MC) between values of Spalt (1958) and FPL 

Adsorption Desorption 

Average of adsorption 

and desorption 

Species 

65%

RH

80%

RH

95%

RH

65%

RH

80%

RH

95%

RH

65%

RH

80%

RH

95%

RH

Picea glauca –0.1 –0.2 –1.6 2.9 3.4 3.2 1.4 1.6 0.8 

Pinus monticola –0.7 –0.9 –2.5 1.6 1.8 1.0 0.5 0.5 –0.8 

Pinus ponderosa –0.6 –0.7 –2.1 1.6 1.8 0.9 0.5 0.5 –0.6 

Tsuga heterophylla 0.4 0.4 –0.5 2.9 3.3 2.3 1.6 1.8 0.9 

Pseudotsuga menziesii –0.2 –0.1 –0.7 2.7 3.3 3.1 1.3 1.6 1.2 

Thuja plicata –1.1 –1.7 –4.0 1.3 0.9 –1.9 0.1 –0.4 –2.9 

Sequoia sempervirens –1.1 –1.6 –3.6 1.3 0.6 –2.5 0.1 –0.5 –3.0 

Pinus caribaea –1.1 –1.3 –2.0 1.7 2.0 1.4 0.3 0.4 –0.3 

Tilia americana –1.5 –1.3 –0.3 1.5 2.2 2.5 0.0 0.5 1.1 

Ceiba pentandra –0.3 0.2 1.5 2.1 3.0 4.7 0.9 1.6 3.1 

Jacaranda copaia –0.4 –0.5 –1.2 2.5 3.4 4.6 1.0 1.5 1.7 

Dicorynia paraensis –0.2 –0.4 –1.7 2.7 3.4 3.3 1.2 1.5 0.8 

Swietenia sp. –0.6 –1.1 –3.1 2.2 1.8 –1.1 0.8 0.4 –2.1 

Tectona grandis –1.8 –2.5 –4.8 0.5 –0.1 –3.2 –0.6 –1.3 –4.0 

Calophyllum brasiliense –0.2 –0.8 –3.1 3.2 3.4 1.7 1.5 1.3 –0.7 

Licaria cayennensis –2.2 –3.1 –5.9 0.9 0.2 –3.3 –0.6 –1.4 –4.6 

RMSE, all species 1.0 1.3 2.9 2.1 2.5 2.8 0.9 1.2 2.2 
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

29

We used the complete table of dry-bulb temperature, relative 

humidity, and EMC values as presented in Table 5 (identical 

to the 1955 Wood Handbook).6 We used decimal values for 

relative humidity and moisture content, and we converted 

Fahrenheit temperature to absolute (Rankine) temperature 

prior to fitting. The parameters for each model were optimized using the surface-fitting toolbox in Matlab®. For each 

model, we recorded RMSE and coefficient of determination 

(R2) and plotted residuals in 3-dimensional space. Our intention was not to find the model with the best statistics; rather, 

our goal was to identify whether models simpler than HH12 

could fit the data sufficiently well, meaning that they do not 

exhibit trends in the residuals (i.e., larger errors at extreme 

temperatures or moisture contents).

As shown in Table 5, the number of RH/EMC values at a 

given dry-bulb temperature increases with increasing temperature (up to 115 °F, above which the number of values is 

constant). For the purpose of curve fitting, this effectively 

gives more weight to values at high temperatures. We therefore compared two fitting methods: 1) the RH/EMC values 

were not weighted (they were taken directly from Table 

5); and 2) the RH/EMC values at each temperature were 

weighted by the reciprocal of the number of values at that 

temperature. Across all models, these two methods gave 

fitting parameters that were largely the same (they differed 

by less than 10%), and the residuals were visually similar 

6Table 38 in the 1955 Wood Handbook (FPL 1955) contains a typographical 

error: the RH value given as “50” for a dry-bulb temperature of 40 °F and a 

wet-bulb depression of 5 °F should actually be “60”.

in both cases. We suggest that weighting the data at each 

temperature is unnecessary not only because it makes little 

difference but also because it implies we know more about 

the data than we actually do. We have argued previously 

(Section 3.4) that the evidence is inconclusive regarding 

whether the values in the table represent measured data or 

were generated by interpolation and extrapolation, based on 

measured data points that are unknown.

The residuals from four of the seven models tested are 

plotted in Figure 19, and RMSE and R2 values are listed in 

Table 12. While the coefficients of determination for all of 

the models are high, the two- and three-parameter models 

contain noticeable trends in the residuals. The H4 model 

gives better statistics than the GAB6 model, even though 

the latter has more parameters. The H4 model shows minor 

trends in the residuals: they are consistently negative at 

lower temperatures and at high temperatures in conjunction 

with moderate RH levels, and consistently positive at moderate temperatures. The HH12 model gives the best statistics 

and shows the most random distribution of residuals.

In summary, although the HH12 equation provides an excellent fit to the FPL EMC data, physical meaning should not 

be assigned to its parameters. Many different equations are 

able to adequately fit the historic FPL temperature/RH/EMC 

values with fewer than 12 parameters. The H4 model fits the 

data well (nearly as well as the HH12 model), shows only 

minor trends in the residuals, and is easily invertible. For 

these reasons, the H4 model should work well in practical 

applications. This model with its parameters is given below. 

Table 12—Equations used for fitting FPL EMC data and related statisticsa

Abbreviationb

 References Equationc R2 RMSE

B2 Bradley 1936 m  AlnBT lnh 0.934 0.0125 

H2 Henderson 1952   B

AT

h

m 



 



   ln 1 0.952 0.0178 

CM3 Oswin 1946; Chen and Morey 

1989  

C

h

h m A BT 



 





  1

0.979 0.0066 

CP3 Chung and Pfost 1967; 

Pfost et al. 1976 

m  A BlnC T lnh 0.977 0.0070 

H4d Henderson 1952; Zuritz et al. 

1979     D CT B

c m  AT 1T T ln 1 h

0.9998 0.0034 

GAB6 Anderson 1946; Lewicki 2009    

  Ch   F T     B   E T Ch   F T

ABCh D E F T m 1 exp 1 exp 1 exp

exp

  

   0.9996 0.0041 

HH12 Hailwood and Horrobin 1946; 

Simpson 1973 

See Equations (2) and (3) 0.9999 0.0013 

a R2

 is coefficient of determination; RMSE is root-mean-square error in fractional moisture content. b

 Abbreviations consist of letters referring to names followed by the number of adjustable parameters. c

 For ease of illustration, the fractional moisture content (m) is written as a function of fractional relative humidity (h) and absolute temperature (T). Each 

adjustable parameter is given a capital letter starting with “A”; parameters may be positive or negative. d Tc is the critical temperature of water (1164.8 Rankine or 647.1 Kelvin).
General Technical Report FPL–GTR–229

30

For T in degrees Rankine ([°R] = [°F] + 459.67):

(5a)

For T in kelvins ([K] = [°C] + 273.15):

 

(5b)

where h is relative humidity (decimal) and m is equilibrium 

moisture content (decimal).

The inverted form of this model is given below. For T in 

degrees Rankine ([°R] = [°F] + 459.67):

 

(6a)

For T in kelvins ([K] = [°C] + 273.15)

 

(6b)

On the basis of this analysis, we suggest that future editions 

of the Wood Handbook give the H4 equation in addition to 

the HH12 equation. We further suggest that future editions 

of the Wood Handbook provide caveats regarding the table 

of temperature/RH/EMC values. As discussed previously 

(Section 5), the precision with which EMC values have been 

given in Tables 7–9 (to 0.1% MC) is potentially misleading: 

it conceals the variability arising from hysteresis and other 

sources, and it far exceeds the precision required for most 

practical purposes.

7. Conclusions

Our primary aim in this report was to investigate whether 

the methods by which the FPL EMC data were obtained 

(a) (b)

(c) (d)

Figure 19. Residuals (in fractional moisture content) as a function of temperature (°F, x-axis) and fractional relative humidity (y-axis) for four models (see Table 12): B2 (a), CP3 (b), H4 (c), and HH12 (d). Note the difference in scale between 

(a)-(b) and (c)-(d).
Investigation of Historic Equilibrium Moisture Content Data from the Forest Products Laboratory

31

are sufficiently documented to allow the data to be used 

for scientific purposes such as thermodynamic analysis and 

evaluation and validation of physical models. In addition we 

have raised several questions related to practical application 

of the data.

The FPL-generated EMC data set is unreliable with respect 

to uncovering a deeper understanding of wood–water relationships for three reasons: lack of proper documentation 

of methodology; the unsolvable problem of knowing which 

values are determined from direct observations and which 

are interpolated; and the absence of definitive measurement 

error analysis.

Despite these problems, we affirm that the data are useful 

for practical applications such as lumber drying, conditioning of wood specimens prior to testing physical or mechanical properties, and modeling of moisture content of wood 

members in buildings. Comparison of the FPL data set with 

values from the broader literature confirms that the FPL data 

represent “a suitable and practical compromise for use when 

the direction of sorption is not always known” (FPL 1999b). 

However, assuming that the reported level of precision (to 

0.1% MC) is meaningful would be misguided. Although this 

level of precision is useful for showing trends in EMC with 

RH and temperature, the variability in EMC stemming from 

sorption hysteresis and differences between species is considerably larger.

Although the HH12 equation provides an excellent fit to the 

FPL EMC data set, physical meaning should not be assigned 

to its parameters. Many different equations are able to fit the 

historic FPL EMC/temperature/RH values adequately for 

practical purposes.

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